8.3.10 Error Sources

Most modern signal-conditioning systems calibrate errors at room temperature. However, calibration of errors that result from a change in temperature is normally difficult and costly. Therefore, these errors must be minimized by choosing high-precision components such as the INA827 that have improved specifications in critical areas that effect overall system precision. Figure 60 shows an example application.

Figure 60. Example Application With G = 10 V/V and 1-V Differential Voltage

Resistor-adjustable INAs such as the INA827 yield the lowest gain error at G = 5 because of the inherently well-matched drift of the internal resistors of the differential amplifier. At gains greater than 5 (for instance, G = 10 V/V or G = 100 V/V) gain error becomes a significant error source because of the resistor drift contribution of the feedback resistors in conjunction with the external gain resistor. Except for very high gain applications, gain drift is by far the largest error contributor compared to other drift errors (such as offset drift). The INA827 offers the lowest gain error over temperature in the marketplace for both G > 5 and G = 5 (no external gain resistor). Table 2 summarizes the major error sources in common INA applications and compares the two cases of G = 5 (no external resistor) and G = 10 (with a 16-kΩ external resistor). As shown in Table 2, although the static errors (absolute accuracy errors) in G = 5 are almost twice as great as compared to G = 10, there is a great reduction in drift errors because of the significantly lower gain error drift. In most applications, these static errors can readily be removed during calibration in production. All calculations refer the error to the input for easy comparison and system evaluation.

Table 2. Error Calculation

ERROR SOURCE	ERROR CALCULATION	INA827
		SPECIFICATION	G = 10 ERROR (ppm)	G = 1 ERROR (ppm)
ABSOLUTE ACCURACY AT +25°C
Input offset voltage (µV)	VOSI / VDIFF	150	150	150
Output offset voltage (µV)	VOSO / (G × VDIFF)	2000	200	400
Input offset current (nA)	IOS × maximum (RS+, RS–) / VDIFF	5	50	50
CMRR (dB)	VCM / (10CMRR / 20 × VDIFF)	94 (G = 10), 88 (G = 5)	200	398
Total absolute accuracy error (ppm)			600	998
DRIFT TO +105°C
Gain drift (ppm/°C)	GTC × (TA – 25)	25 (G = 10), 1 (G = 5)	2000	80
Input offset voltage drift (μV/°C)	(VOSI_TC / VDIFF) × (TA – 25)	5	200	200
Output offset voltage drift (μV/°C)	[VOSO_TC / ( G × VDIFF)] × (TA – 25)	30	240	240
Total drift error (ppm)			2440	760
RESOLUTION
Gain nonlinearity (ppm of FS)		5	5	5
Voltage noise (1 kHz)		eNI = 17 eNO = 250	6	6
Total resolution error (ppm)			11	11
TOTAL ERROR
Total error	Total error = sum of all error sources		3051	1769